1. Technical Field of the Invention
The present invention relates generally to the field of photonic crystals, and, more particularly, to a photonic crystal interferometer, and to a method for controlling a photonic crystal interferometric switch.
2. Description of Related Art
Photonic crystals (PC) are periodic dielectric structures that can prohibit the propagation of light in certain frequency ranges. More particularly, photonic crystals are fabricated to have spatially periodic variations in refractive index; and for a sufficiently high refractive index contrast, photonic bandgaps can be opened in the structure""s optical spectrum. These bandgaps comprise frequency ranges within which the propagation of light through the photonic crystal is prevented (the term xe2x80x9clightxe2x80x9d as used herein is intended to include radiation throughout the electromagnetic spectrum, and is not limited to visible light).
It is known that introducing defects in the periodic structure of the photonic crystal allows the existence of localized electromagnetic states that are trapped at the defect site, and that have resonant frequencies within the bandgap of the surrounding photonic crystal material. By providing a region of such defects extending through the photonic crystal, a waveguiding structure is created which can be used in the control and guiding of light.
A photonic crystal that has spatial periodicity in three dimensions can prevent the propagation of light within the crystal""s bandgap in all directions; however, the fabrication of such a structure is technically challenging. A more attractive alternative is to utilize a 2-dimensional photonic crystal slab that has a two-dimensional periodic lattice incorporated within it. In a structure of this sort, light propagating in the slab is confined in the direction perpendicular a major surface of the slab via total internal reflection, and light propagating in the slab in directions other than perpendicular to a major surface is controlled by the properties of the photonic crystal slab. A two-dimensional photonic crystal slab has the advantage that it is compatible with the planar technologies of standard semiconductor processing; and, in addition, the planar structure of the slab makes an optical signal in a waveguide created in the slab more easily accessible to interaction. This provides the additional advantage that the structure is susceptible to being used to create active devices.
Both theoretical and experimental work have demonstrated the efficient guidance of light in a two-dimensional photonic crystal slab waveguide device (see xe2x80x9cDemonstration of Highly Efficient Waveguiding in a Photonic Crystal Slab at the 1.5 xcexcm Wavelengthxe2x80x9d, S. Lin, E. Chow, S. Johnson and J. Joannopoulos, Opt. Lett. 25, pp. 1297-1299, 2000). Furthermore, experimental work is demonstrating the capability of fabricating such devices that are able to propagate light with a high degree of efficiency. As a result, there has already been some investigation into potential applications for interacting with the guided optical modes of the waveguide device. Such applications, which have been previously discussed include static (fixed wavelength) or tunable channel drop filters, and tunable resonant microcavity defects (see U.S. Pat. No. 6,058,127).
Commonly assigned, copending U.S. patent application Ser. No. 09/846,856, the disclosure of which is incorporated by reference herein, describes a photonic crystal waveguide apparatus which includes a transverse resonant stub tuner. This apparatus has transmission zeros at specified frequencies determined by the resonant frequencies of the transverse stub tuner (the term xe2x80x9ctransmission zeroxe2x80x9d refers to a frequency range within the bandgap of the photonic crystal at which light that is otherwise capable of being transmitted by the waveguide is prevented from being transmitted). The application also discloses techniques for tuning electromagnetic properties of the transverse resonant stub tuner to shift the transmission zeros in the frequency domain of the waveguide apparatus. These techniques permit the apparatus to be utilized in applications such as a 1xc3x971 optical switch and an optical modulator, relying on the ability of the transverse resonant stub tuner to modify the magnitude of the optical power in the waveguide apparatus.
The present invention provides a photonic crystal interferometer apparatus for controlling the interference of light in a waveguide of the apparatus.
An exemplary photonic crystal interferometer apparatus according to an embodiment of the present invention may comprise a photonic crystal with a waveguide in the photonic crystal. The waveguide may comprise at least one input portion, at least two output portions and an interference channel connecting the at least two output portions. The waveguide is capable of transmitting light within a bandgap of the photonic crystal. There is a resonant member connected to at least one of the at least two output portions to control a property of light in the waveguide to control interference of light in the waveguide.
According to a another embodiment of the invention, the resonant member comprises a resonator region and a connecting channel connecting the resonator region and at least one of the at least two output portions. By setting the resonant frequency of the resonator region, the resonator region and the connecting channel cooperate with the interference channel to cause light in one of the at least two output portions of the waveguide to constructively interfere and light in another of the at least two output portions to destructively interfere with light in the interference channel.
According to a second embodiment of the invention, the waveguide comprises a region of first defects in a periodic lattice of the photonic crystal which extends through the photonic crystal, and the connecting channel comprises at least one second defect in the periodic lattice. The resonator region comprises a region in the photonic crystal in which the periodic lattice is modified in an appropriate manner to define the resonator region.
According to a third embodiment of the invention, the at least one input portion of the waveguide comprises one input portion and the at least two output portions comprise two output portions. The periodic lattice of the photonic crystal comprises an array of dielectric posts, and the waveguide is created by omitting portions of lines of the posts. Specifically, the input portion is created by omitting a portion of a line of posts, and the two output portions are each created by omitting portions of two lines of posts that are perpendicular to each other, and which are connected to the input portion. The interference channel is created by omitting a portion of a line of posts that connects the two output portions. The connecting channel is created by omitting one additional post in the periodic lattice to define a short channel that is connected to one of the two output portions and extends perpendicularly from a sidewall of the one output portion. The resonator region comprises a generally square region having a sub-array of posts that are larger in diameter than the other posts in the periodic lattice.
By controlling parameters of the resonator region, such as the number of posts in the region and the size of the posts; the resonant frequency of the resonant member, and, hence, the phase of the light in one of the two output portions of the waveguide can be effectively controlled. By controlling the phase of the light in the one of the two output portions, light interference is created in the two output portions. The interference can be constructive interference in one of the two output portions, and destructive interference in the other of the two output portions, thus providing a photonic crystal interferometer.
According to a fourth embodiment of the invention, the photonic crystal interferometer apparatus comprises a photonic crystal interferometric switch. In particular, constructive interference in an output portion allows the light to propagate through the portion, while destructive interference in an output portion prevents the light from propagating though the portion. The present invention utilizes this feature by providing a tuning member connected to the resonator region of the resonant member for tuning the resonant frequency of the resonator region. The tuning member preferably comprises a dielectric constant tuning member that controls the dielectric constant of the dielectric posts in the resonator region; which, in turn, controls the resonant frequency of the resonator region. By operation of the tuning member, the constructive and destructive interference in the two output portions can be reversed, thus creating an optical switch.
The dielectric constant tuning member can be an electronic dielectric constant tuning member for tuning the dielectric constant using, for example, the charge carrier effect or the electro-optic effect. Alternatively, the tuning member can be an optical dielectric constant tuning member for tuning the dielectric constant using, for example, the photorefractive effect.
According to further embodiments of the invention, the photonic crystal interferometric switch can, for example, be operated as an nxc3x97m switch, where n is the number of input portions, and m is the number of output portions. Furthermore, the photonic crystal interferometric switch can be operated as a modulator.
In general, the photonic crystal interferometric switch of the present invention can be utilized in numerous applications and provides a designer with the capability of designing complex routing architectures in an optical integrated circuit.
Yet further advantages and specific features of the invention will become apparent hereinafter in conjunction with the following detailed description of exemplary embodiments of the invention.